Why not use a Reference line? (see Properties->Presentation->Reference Lines) They're made for this and add a clear indicator without rearranging your object or taking away the focus from the actual values. Also, a reference line will behave better when a lot of brand values are displayed, while an additional bar may disappear altogether...

Add an expression like =avg(Value), and a suitable line text. The label can include the numerical value if you create a label expression like: